1. Field of the Invention
The present invention relates to a device for controlling amounts of exhaust gas recirculation and idle speeds of an internal combustion engine.
2. Description of the Prior Art
The exhaust gas recirculation system (which is referred to as "EGR" hereinafter) is a method of purifying exhaust gases of internal combustion engines, which intends to re-introduce a part of the exhaust gas into an intake air flow to restrain temperature rise in a combustion chamber, thereby reducing the generation of nitrous oxides (NOx).
Recently, moreover, it has been required to control idling (no-load) revolutions precisely in order to improve the purification of the exhaust gases and to decrease the fuel consumption. (The idle speed control is referred to as "ISC" hereinafter.)
In carrying out the above EGR and ISC, the recirculation and idle speed have been separately controlled. Referrring to FIG. 1, intake air is supplied through an intake air tube 1 to an engine 2 as shown in broken line arrows. A part of the exhaust gas is re-introduced into the intake air tube 1 through an EGR tube 4 branched off an exhaust tube 3 and an EGR control valve 5 provided in the EGR tube 4 as shown in solid line arrows.
On the other hand, upstream and downstream sides of a throttle valve 6 provided in the intake air tube 1 are connected by a bypass 7 including an ISC control valve 8.
Intake negative pressure downstream of the throttle valve 6 is withdrawn through a negative pressure tube 9 and fed through constant pressure valves 10 and 11 to the EGR control valve 5, solenoid valve 12, ISC control valve 8 and solenoid valve 13.
The solenoid valves 12 and 13 close and open in response to control signals S.sub.5 and S.sub.6 from a control circuit 14 to shut off the negative and atmospheric pressures alternately. Accordingly, the negative pressures supplied to the EGR control valve 5 and ISC control valve 8 vary to change open areas of these control valves, thereby enabling the amounts of EGR and intake air flow rate when idling to be controlled.
The control circuit 14 consists of, for example, a micro computer, to which various signals corresponding to the operating parameters of the engine are supplied, such as intake air flow signals S.sub.1, engine revolution signals S.sub.2, engine temperature signals S.sub.3, throttle valve open area signals S.sub.4 and the like. Then the control circuit effects required arithmetic processes for these signals to generate control signals S.sub.5 or S.sub.6 for optimum amounts of EGR or intake air flow rate.
With the hitherto used devices, as above described the EGR and ISC are controlled in completely separate systems each having one solenoid valve. As these solenoid valves are required to close and open exactly in high responsibility according to given control signals, they become inevitably precise and expensive to increase the cost of the entire device.
In order to solve this problem, a system has been proposed wherein the EGR and ISC are changed to be controlled by one solenoid. In this case, the ISC is effected only when a throttle valve is closed (when idling) and the EGR is effected only at the time other than the above time because the amount of NOx is little to an extent not requiring the EGR when idling. Therefore, there is provided with one solenoid valve and a change-over valve for supplying the negative pressure regulated by the solenoid valve to any one of the ISC and EGR control valves at the time when the throttle valve is completely closed or other than the time, thereby enabling only one solenoid valve to control the EGR and ISC.
When a throttle valve is closed to the most possible extent, there is generally a clearance between a periphery of a valve body and a wall of an intake air passage. It will be therefore understood that the expression "throttle valve is closed" used herein means the closed position of a throttle valve when idling.
With the above system, while the EGR control valve is opened when the change-over valve is changed from the EGR to ISC, the EGR control valve is kept opened, so that the EGR is effected during idling of the engine and therefore the revolutions of the engine tends to be unstable.